Magnetrons



y 1959 J. E. BAKER ET AL 2,887,615

- MAGNETRONS Filed Jan. 16, 1956 Fan INVENTORS 4/omv fiuwuv 4x51:

Wr-roR'JeY United States PatentO 1 ce MAGNETRONS Application January16,1956,SerialNo.559,423,

3 Claims. (c1. sis-39.77

This invention relates to magnetrons of the type having an anode in theform of a metal block with parallel end I faceswhich is piercedperpendicular to said endfaces both by a cylindrical electron orbithole, within which is supported the cathode of the magnetron, and by aplurality of slots opening into said hole, which slotsconstitute aplurality of cavity resonators separated by anode segments evenly spacedaround the axis of said electron orbit hole, and including outputcoupling means which introduces into the anode resonator system a planeof symmetry containing the axis of the electron orbit hole. Such outputcoupling means may consist, for example, of a coupling loop projectinginto a single cavity resonator or of an extension of one iof said cavityresonator slots to the exterior of the anodeblock as part of a waveguideoutput system. The term metal block does not necessarily imply that theblock is constructed in one piece-it may in some cases consist of anumber of parts soldered or otherwise attached together; also the blockneed not necessarily be solid-it may, for example, contain channels forthe passage of cooling fluid.

In magnetrons of this type the anode is usually provided at each endface with two concentric coupling rings of which one ring is connectedto each of one set of alternate anode segments and the other ring isconnected to each of the other, interleaved, set of anode segments. Wehave found that advantages are obtained if each of the said fourcoupling rings is broken at one place, and at each end of the anode thebreaks in the two coupling rings at that end are disposed on radii whichare on opposite sides of and are equally inclined to the plane ofsymmetry introduced into the anode resonator system by the outputcoupling means. Preferably the output coupling means is associated witha single cavity, for example as a coupling loop entering the cavity oras a slot extending from the back of the cavity to the exteriorof theblock as part of a wavegulide output system, and the breaks in thecoupling rings are made each on the side of the anode opposite to saidsingle cavity and where the ring passes over that one of the anodesegments to which it is not connected which lies nearest to said planeof symmetry, said plane passing through said single cavity.

The advantages obtainable by this construction are an increase in thestability of 1r-mode oscillation of the magnetron and/ or a quicker rateof build-up of oscillations in the wanted ar-mode when the magnetron issubjected to pulsed operation. 7

In such a magnetron the anode supporting structure usually has the formof a metal block recessed from opposite end faces to form a centralpartition with end spaces on each side thereof, the central partitionbeing pierced by the electron orbit hole and resonator slots to providethe anode.

We have further found that in a magnetron of this form the end spacesare coupled with the anode resonator system, so that. the end spaces andthe anode system resonate together as a whole, and=we have discoveredthat the frequency of this resonance, and hence the output frequency ofthe magnetron, can be simply controlled over an appreciable range byvarying the internal dimensions of the end spaces.

In addition by suitable adjustment of the dimensions of the end spaces,the advantages as regards vr-mode stability and quick rate of build-upof oscillations into the vr-mode, resulting from the presence of breaksin the coupling rings at each end of the anode, can be enhanced. Toexplain this, it may be stated that the main cause of instability ofrr-mode oscillation of known magnetrons has resulted from the factthatin pulse operation the magnetron usually starts to oscillate, at thebeginning of each pulse, in the (1r-1) mode and then changes over to thevr-mode as the anode current increases; in some cases, however,operation in the unwanted (1r-1) mode persists to high current levels,with the result that operation in the 1r-mode is impaired, oi may evenbe absent altogether. The (1r1) mode is driven by its reverse componentwhose threshold voltage lies below that of the ir-mode so that it isexcited before the ir-mode.

" We have found that the effect of the discontinuities introduced by thecoupling ring breaks provided at each endof the anode is to alter therelative amplitudes of the space harmonics contained in the spacepattern of the electric field set up by the oscillating anode system. Wehave further found that by varying the dimensions "of at least one ofthe end spaces, the amplitudes of the space harmonics of the electricfield pattern can be con trolled so that a relatively large amplitude isobtained for the +1) space harmonic of the 'lF-lIlOdC field pattern, andthat as the threshold voltage of the said (n+1) space harmonic of theIr-mode field, which is below the threshold voltage of the reversecomponent of the unwanted (-rr-l) mode, is reached at the beginning ofeach operating pulse. The anode system starts to operate at the 1r-modefrequency on this vr-mode field harmonic; because the ir-mode fieldpattern is thus established, further increase of the yoltage then causesthe anode system to change readily into the fundamental 1r-modeoperation without change of frequency and with little or no interferencefrom the (1r- 1) mode. Both the starting and continuation ofoscillations at the 1r-mode frequency are thereby facilitated.Accordingly, therefore, the present invention provides a method ofmanufacture of a magnetron of the kind referred to wherein the anodesupporting structure has the form of a metal block recessed from eachend face to form a central partition with end spaces on each sidethereof, the central partition being pierced by the electron orbit holeand resonator slots to provide the anode, and being provided at each endface with two concentric coupling rings each broken at one place, whichmethod ofmanufacture includes the step of adjusting the internaldimensions of at least one end space so as to increase the amplitude ofthe (1+1) harmonic of the space pattern of the electrical field set upby the anode system when oscillating in the ir-mode to obtain thedimensions at Which the amplitude of the said harmonic is sufiicientlylarge to ensure that reliable oscillation of the magnetron in the1r-m0de is obtained in pulse operation of the magnetron.

In carrying out a manufacture in accordance with the invention, thespace pattern of the electrical field-distribu tion set up by the anodesystem when oscillatingin the ir-mode can, if required, be determinedexperimentally, and the amplitude of the (n+1) harmonic ascertained by aFourier analysis, for each adjustment of the end space dimensions, butas this is a tedious process it will inpractice usually be moreconvenient to judge theeflfect of a variation in the end spacedimensions byobserving Patented May 19, 1959 3 the performance, asregards mode stability, of the magnet ron by "measurement of its outputfrequency, the end space dimensions being regarded as suitably adjustedwhen good mode stability is obtained.

Our present experience is that the amplitude of the said (n+1) harmonicshould preferably be about 10% of the amplitude of the fundamental, butthis is not critical and may be much less in some cases.

' It will be appreciated that the main utility of the invention lies inthe adjustment of the end space dimensions in the design of a prototypeof a magnetron, which may involve the construction of a number ofmagnetrons of the same kind which have different values of the said endspace dimensions; once the optimum dimensions have been obtained, theconstruction of numbers of magnetrons to the established designincluding the optimum dimensions can then proceed without need forindividual adjustment of this kind in their construction.

, -It will also be appreciated that an adjustment of the end spacedimensions will also alter the operating frequency of the magnetron, butallowance can be made for this in the design of the magnetron by asuitable adjust ment of the dimensions of the anode resonant cavities;we have found, in general, that a change in the frequency of the1r-1'I1Od6 resulting from the adjustment of the end space dimensionsproduces also a substantially equal change in the frequency of theunwanted mode, so that thedi sturbing effect of the unwanted mode is notincreased by its frequency separation from the 1r-mode being reduced.

We 'found that the most convenient way of adjusting the end spacedimensions in a method of manufacture in accordance with the inventionis to vary the effective internal diameter of the end space, and thatthis can be conveniently adjusted by altering the depth and/or width ofan annular groove, formed in the side-wall bounding the end space,concentric with the axis of the end space.

One magnetron which has been manufactured by a method in accordance withthe invention is illustrated by way of example in the accompanyingschematic drawing, which shows a section in a plane containing the axisof the magnetron, and the invention will be further described withreference to this drawing.

The magnetron shown in the drawing has a rectangular copper 'anode block1 provided round threesides with a channel 2 for thepassage of coolingfluid. The block is pierced between its two major end faces by acylindrical aperture which is of somewhat smaller diameter at the centreof the block to form a seating for the attachment ofradial copper anodesegments 3 the spaces between whichform the anode cavity resonators andthe opposed inner ends of which define the cylindrical electron orbitspace of the magnetron. The anode segments effectively form a centralpartition in the anode block having cylindrical end spaces 4 and 5 oneach side thereof.

Into one side of the block is spigotted, radially with respect to theanode, a cylindrical metal annulus 6 the space within which communicateswith one of the anode cavity resonators through two holes 7 and 8 in theside of the block, along which holes pass output leads 9 and 10 whichterminate in coupling loops 11 and 12 within the said resonator, theloops being oriented oppositely with respectto'each other. The leads 9and 10 are continued by sleeves 13 and 14 into which they slidably fitand which are connected together at their outer ends by a bridge piece15 to form an output coupling loop. The loop is supported by a sleeve 16extending from the centre of the bridge piece slidably over a supportrod 17 spigotted into a bushing 18 fitted into the bottom part ofthemnulus 6. The sleeves 13, 14 and 16 permit the length of theoutputloop to be adjusted to the optimum value in assemblyof themagnetron andare thereafter fixed to theleads 9 and 10 and rod 17, respectively. Theupper part of the annulus 6 is attached to a cylindrical metal flange19-which ishermetically closed bya glassto-metal dome seal 20 of knownkind and also carries a metal cylinder 21 which surrounds the dome 20and forms part of a waveguide output connection.

The cathode of the magnetron consists of a hollow metal tube 22 having acentral region of reduced outer diameter forming a recess .in which isprovided the electron emissive material 23. The tube 22 is of expandedinternal and external diameter at each end and is covered on itsexternal surface at each of these ends with a thin skin 24 of a metal ofhigh thermal emissivity. The tube 22 is supported at one end from ametal tube 25 which is in turn supported from an end plate 26, closingthe corresponding side of the anode block, by means of the glass sleeve27 sealed at one end to the tube 25 and at the other end to a metal tube28 spigotted into the end plate 26.

The cathode heater 29 is arranged within the cathode tube 23 and issupported at one end by a metal rod 30 which passes along the tube 25and is in turn supported therefrom by the insulating spacer 31 andglass'button 32 which seals the space between the rod 3%) and tube 25.The other end of the heater is supported from an annular insert ofmagnetic material 33 which is fitted into the. expanded end of the tube23 for improving the uniformity of the operating magnetic field withinthe electron orbit space in use of the magnetron. A similar insert 34 isprovided within the other expanded end of the tube 23, for the samepurpose. The tube 25 and rod 30 provide also the cathode and heatersupply connectrons.

'The'side of the anode block opposite to that through which the cathodeand heater support and supply means aretaken -is closed by theattachment thereto of an end plate 35 into the centre of which isspigotted a metal domef36 which extends over the corresponding end ofthe cathode tube 23.

The space within the magnetron is evacuated and sealed off by-means ofan exhaust stem (not shown), holes 37 and 38 being provided through thewall of the anode block between the end spaces 4, 5 and the interior ofthe output means for facilitating the exhaustion in manufacture of themagnetron.

The anode segments 3 are provided at each end with two concentricmode-locking rings 39, 40 and 41, 42 respectively, which areea'ch'broken at one point; the elfect of these breaks is to include theend spaces 4 and 5 within the. resonant anode system and. in accordancewith the" present invention the effect of these end spaces is controlledfor obtaining good stability of pulse operation of the magnetron in the1r-mode by providing in the wall of the block bounding each end space anannular groove 43 and 44 respectively whose width and depth have optimumvalues as determined by trial in the design of the magnetron, aspreviously explained.

'We. claim:

1. In the manufacture of a resonant cavity magnetron having an anodesupporting structure in the form of a metal block recessed from each endface to form a central partition with cylindrical end spaces on eachside thereof, the central partition being pierced by the electron orbithole and a plurality of resonator slots to provide the anode, thecathode of the magnetron being supported Withinthe electron orbit hole,an output coupling means beingprovided on one side of the anode so as tointroduce into the anode system a plane of symmetry containing the axisofthe electron orbit hole, and the anode being provided at each end facewith two concentric coupling rings of which one ring is connected toeach of one set ofalternate .anode segments and the other ring isconnected to each of the other set of anode segments, each of the saidfour coupling rings being broken at one place, and at each .end of theanode the breaks in the two couplingrings at that-end being disposed onradii which are on opposite sides of and are equally inclined to theplane-of symmetry.- introduced intothe anode resonator system by theoutput coupling means: that improvement including the step of providingan annular groove coaxial with the electron orbit hole in thecylindrical side wall of at least one end space and adjusting thedimensions of the groove so as to increase the amplitude of the (77+1)harmonic of the space pattern of the electrical field set up in theelectron orbit space by the anode system when oscillating in the 1r-modeuntil groove dimensions are obtained at which the amplitude of the saidharmonic is sufficiently large to ensure that reliable oscillation ofthe magnetron in the 1r-mode is obtained in pulse operation of themagnetron.

2. A method according to claim 1 wherein together with adjustment of thedimensions of at least one annular groove in an end space there iseffected a concurrent adjustment of the dimensions of another part ofthe anode resonator system for maintaining the same operating frequencyfor the magnetron.

3. A resonant cavity magnetron having a sealed evacuated envelopecontaining an anode supporting structure in the form of a metal blockrecessed from each end face to form a central partition with cylindricalend spaces on each side of the partition, the central partition beingpierced by a coaxial hole which provides the electron orbit hole and bya plurality of slots extending radially from said hole which provide theanode resonators, a cathode supported coaxially within the electronorbit hole, conducting paths to the anode and cathode extending to theexterior of the envelope, an output coupling means provided on one sideof the anode which introduces into the anode system a plane of symmetrycontaining the axis of the electron orbit hole, on each end face of theanode two concentric coupling rings of which one ring is connected toone set of alternate anode segments and the other ring is connected toeach of the other set of anode segments, each of the said four couplingrings being broken at one place, and at each end of the anode the breaksin the two coupling rings at that end being disposed on radii which areon opposite sides of, and are equally inclined to, the plane of symmetryintroduced into the anode resonator system by the output coupling means,and in the cylindrical side wall of at least one end space a coaxialannular groove of dimensions critically adjusted to produce vr-modeoscillation of the magnetron in pulse operation without any component ofthe (1r-1) mode frequency appearing in the magnetron output.

References Cited in the file of this patent UNITED STATES PATENTS2,447,537 Ronci Aug. 24, 1948 2,466,922 Wax Apr. 12, 1949 2,477,317Spencer July 26, 1949 2,550,614 Spencer Apr. 24, 1951 2,607,019 DoddsAug. 12, 1952 2,625,669 Anderson Jan. 13, 1953

